Composite parts and structures such as those used in the automotive, marine and aerospace industries may be fabricated using automated composite material application machines. For example, automated fiber placement (AFP) machines may be used to produce generally cylindrical or tubular composite parts by wrapping relatively narrow strips of slit composite tape or “tows”, collimated into a wider band around a rotating manufacturing tool, such as a mandrel. AFP machines are typically operated by NC (numeric control) or CNC (computer numerical control) controllers which control movement of application heads and ancillary functions, including placing and cutting the fiber tows “on the fly”.
In a typical AFP machine application, carbon fiber-epoxy tows are pulled from storage spools or creels in a refrigerated creel house through a set of guides. From the guides, the tows enter a cutter assembly where they are cut to the correct length by knives as the material course, also called a tow band, is laid over a substrate. The substrate may comprise a tool, a mandrel or one or more underlying layers of composite material which have been previously laid down and compacted. Each tow has a dedicated cutting knife, however the number of knives may vary depending upon the number of tows and the width of each tow. As the tows emerge from the cutter assembly, they pass over a compaction roller which applies and compresses the tows onto the substrate surface. Heat may be applied to the tow immediately before it is placed on the substrate in order to increase the tackiness of the resin. Tension is maintained on the tows at all times to assist in pulling them through the AFP machine.
As a tow band is being laid, it is possible that one or more tows may be missing from the band due to any of several causes. For example, a creel may run out of tow material, or the tow may break under tension, leaving the free segment to be laid down as a partial segment, or may drop off the substrate entirely. Also, the tow may become jammed in transit through the AFP machine, preventing it from being applied to the substrate. It is also possible that the cutting knife may be actuated prematurely, causing the tow to be cut short. In some applications, tow bands may be narrow by design, which requires that one or more tows be intentionally dropped or cut short. In other applications, a tow band may increase in width from a narrow to a wide region, requiring the addition of tows that have been previously cut and blocked from travel. When an event occurs of the type discussed above, or if a tow is misplaced, it is important to determine the cause, preferably in near real-time.
In-process vision inspection systems have been devised for detecting missing and/or misplaced tows. These inspection systems rely on remote observation of the substrate surface using machine vision technology that may incorporate automated image analysis techniques to identify inconsistencies. The systems may include cameras for monitoring the delivery of the tows for possible breaks or jams. However, none of these prior systems may be effective in quickly determining whether the cutter knives are operating properly, or whether inconsistencies in tow placement are related to cutter knife operation.
Accordingly, there is a need for a system that directly monitors cutting knife operation and correlates missing tows and tow misplacement with cutting knife operation. Embodiments of the disclosure are intended to satisfy this need.